This invention relates to the architecture of electronic graphics systems for displaying portions of multiple images on a CRT screen.
In general, to display an image on a CRT screen, a focused beam of electrons is moved across the screen in a raster scan type fashion; and the magnitude of the beam at any particular point on the screen determines the intensity of the light that is emitted from the screen at that point. Thus, an image is produced on the screen by modulating the magnitude of the electron beam in accordance with the image as the beam scans across the screen.
Similarly, to produce a color image on a CRT screen, three different beams scan across the screen in very close proximity to each other. However, those three beams are respectively focused on different color-emitting elements on the screen (such as red, green, and blue color-emitting elements); and so the composite color that is emitted at any particular point on the screen is proportional to the magnitude of the three electron beams at that point.
Also, in a digital color system, the intensity and/or color of the light that is to be emitted at any particular point on the CRT screen is encoded into a number of bits that is called the pixel. Suitably, six bits can encode the intensity of light at a particular point on a black and white screen; whereas eighteen bits can encode the color of light that is to be emitted at any particular point on a color screen.
Typically, the total number of points at which light is emitted on a CRT screen (i.e., the total number of light-emitting points in one frame) generally is quite large. For example, a picture on a typical TV screen consists of 480 horizontal lines; and each line consists of 640 pixels. Thus, at six bits per pixel, a black and white picture consists of 1,843,200 bits; and at eighteen bits per pixel, a color picture consists of 5,529,600 bits.
In prior art graphics systems, a frame buffer was provided which stored the pixels for one frame on the screen. Those pixels were stored at consecutive addresses in the sequence at which they were needed to modulate the electron beam as it moved in its raster-scanning pattern across the screen. Thus, the pixels could readily be read from the frame buffer to form a picture on the CRT screen.
However, a problem with such a system is that it takes too long to change the picture that is being displayed via the frame buffer. This is because 1.8 million bits must be written into the frame buffer in order to change a black and white picture; and 5.5 million bits must be written into the frame buffer to change a color picture. This number of bits is so large that many seconds pass between the time that a command is given to change the picture and the time that the picture actually changes. And typically, a graphics system operator cannot proceed with his task until the picture changes.
Also in a graphics system, the picture that is displayed on the screen typically is comprised of various portions of several different images. In that case, it often is desirable to display the various image portions with different degrees of prominence.
For example, it is desirable for each of the image portions to be displayed in its own independent set of colors and/or be displayed with different blink rates. However, this is not possible with the above-described prior art graphics system since there is no indication in a frame buffer of which image a particular pixel is part of.
Accordingly, a primary object of the invention is to provide an improved graphics system for electronically displaying multiple images on a CRT screen.